[单选题]
A distinctive featurexl3hkzk nz 6fn39v iojq) ;5xr0 v),zg o,7/c**c n: uj/ymy sltxe*t z (ybzdk/f quarks is that they
A. have zero rest mass.
B. have zero charge.
C. have fractional electric charge.
D. are always observed singly, since they do not readily interact with other particles.
